The present invention relates to thick film heaters, such as those used for injection molding nozzles, and more particularly to a metallic overcoat for the termination of the same.
The injection molding industry in particular has long needed improved heating for runner nozzles. A typical injection molding systems has a multitude of heated passage ways carrying melted thermoplastics (xe2x80x9cmeltxe2x80x9d). The melt is delivered into molds via a series of runner nozzles. It is vital that these runner nozzles be kept heated to prevent the melt from freezing too early, before it reaches the mold cavity. If a portion of the melt freezes in the nozzle, the nozzle is incapable of adequately controlling the flow rate of the melt, and may become completely blocked in some instances.
It is well known to place electrical resistance heaters around the runner nozzles to prevent the melt from freezing. Over the years, many new advancements have been made to the heaters to make them more reliable, provide more even heat distribution, and to reduce the size of the heaters. The size of the heater is an important factor, particularly with small mold sizes. Obviously, the smaller the heater around the nozzle, the more nozzles (with heaters) can be placed on the same manifold. Every millimeter saved in the size of the heater thickness can possibly result in several additional nozzle/mold combinations on the same manifold.
Eventually experimentation began involving the use of thick film heating elements to heat the runner nozzles. Only in very recent years was the idea of thick film heating for runner nozzles made feasible and practical by Watlow Electric Manufacturing Company of St. Louis, Mo. Watlow""s improvement is disclosed in U.S. Pat. No. 5,973,296 to Juliano et al. (xe2x80x9cthe ""296 patentxe2x80x9d), which is incorporated herein by reference in its entirety. Essentially, the ""296 patent taught placing the thick film heating element on a metal tubular substrate with a dielectric film on either side of the heating element.
Although the ""296 patent represents a giant advancement in the art of runner nozzle heating, it still has its limitations as disclosed. The most significant limitation of the ""296 patent is the attachment of the power leads. The thick film nozzle heater of the ""296 patent is significantly thinner than any other heaters that could deliver heat as reliably and as evenly. It is significantly thinner than heaters with coiled resistance wire, but the magnitude space conservation is based on the width at thickest portion of the heater design, which is typically the termination of the heating element. The heating element must have an electrical current to operate, and that current must be fed by some sort of power lead. The power leads in the ""296 patent represent the thickest portion of the heater.
Additionally, the biggest limitation in the ""296 heater compared to its coiled resistance wire ancestors is mechanical strength. Coiled resistance wire heaters have an outer sheath, typically made of stainless steel or similar alternative. The thick film heater, although tough in its own right, is more vulnerable to physical damage than a heating element with an outer metal sheath.
Accordingly, it is an object of the present invention to provide a thick film heater with a minimized maximum heater thickness.
It is a further object of the present invention to provide improved mechanical resistance for the heating element.
It is still a further object of the present invention to provide such a heater with improved thermal efficiency.
In view of the above objects, the present invention is a tubular thick film heater with a metallic overcoat placed over at least the termination portion of the heater. The overcoat satisfies, directly and indirectly, all of the above objects. The overcoat obviously provides mechanical safety to the heating element, but in the presented embodiment below, it allows the power leads to be attached to the heating element in a way that does not increase wall thickness of the heater. This actually reduces the total thickness of the heater (even with the overcoat) when compared to a thick film heater with conventionally attached power leads. Lastly, the overcoat directs more heat transfer through the substrate (inner layer), and thus onto the runner nozzle. The added heat transfer (or reduced heat loss to the outside) means a more efficient heater and less energy consumption.
The overcoat is placed around the thick film heater with holes pre-drilled therein to match the termination points of the heating element. This allows the termination points to be seen through the holes in the overcoat. Narrow holes parallel to the axis of the tube are then drilled in the joined assembly. Power leads are then inserted into the narrow holes. The larger holes in the overcoat wall allow the power leads to be properly affixed to the termination points on the heating element using any one of several known methods. Once the heating element is properly terminated, the hole in the overcoat is filled in with a dielectric potting material. Although the overcoat can run the entire length of the heater, it is foreseeable that the overcoat may only cover the termination portion of the heating element.
The resulting heater has a wall thickness on the order of 2 millimeters. More importantly, however, the termination of the heating element and the power leads do not increase the overall heater thickness at all.